build-sys: Turn Rust LTO off by default, add --enable-lto flag

For us, this is primarily right now a size issue.  See:
https://internals.rust-lang.org/t/rust-staticlibs-and-optimizing-for-size/5746

For more information, there are these two issues:
rust-lang/cargo#4349
https://bugzilla.mozilla.org/show_bug.cgi?id=1386371

The basic issue here is that a build with LTO off (and a trivial
change to add a `println!` takes 14s here, and with it on takes 38s.
However, with LTO off the stripped size of `librpmostree_rust.a` is
`6M`, with LTO on it's `1.1M`.

I named this `--enable-lto` as I'd like to investigate doing this
for the C code too.

configure.ac

@@ -228,7 +228,7 @@ fi
 AC_MSG_RESULT($debug_release)
 
 dnl These bits based on gnome:librsvg/configure.ac
-dnl By default, we build in public release mode.
+dnl By default, we build in release mode (but without LTO!)
 AC_ARG_ENABLE(rust-debug,
   AC_HELP_STRING([--enable-rust-debug],
     [Build Rust code with debugging information [default=no]]),
@@ -244,6 +244,19 @@ AM_CONDITIONAL(RUST_DEBUG, [test "x$rust_debug_release" = "xdebug"])
 dnl Unconditional now.
 RPM_OSTREE_FEATURES="$RPM_OSTREE_FEATURES rust"
 
+dnl Only use this for package builds, as it will hackily edit Cargo.toml right now
+AC_ARG_ENABLE(lto,
+AC_HELP_STRING([--enable-lto],
+               [Build code with Link Time Optimization [default=no]]))
+dnl https://github.com/rust-lang/cargo/issues/4349
+dnl https://bugzilla.mozilla.org/show_bug.cgi?id=1386371
+if test "${enable_lto}" = yes; then
+    if ! grep -q '^lto.*=true' rust/Cargo.toml 2>/dev/null; then
+        echo '# Inserted by configure --enable-lto' >> rust/Cargo.toml
+        echo "lto = true" >> rust/Cargo.toml
+    fi
+fi
+
 dnl And propagate the release/debug type to cmake
 cmake_args=-DCMAKE_BUILD_TYPE=RelWithDebugInfo
 if test ${debug_release} = debug; then
@@ -286,6 +299,6 @@ echo "
     introspection:                           $found_introspection
     bubblewrap:                              $with_bubblewrap
     gtk-doc:                                 $enable_gtk_doc
-    rust:                                    $rust_debug_release
+    rust:                                    $rust_debug_release (lto: ${enable_lto:-no})
     cbindgen:                                ${cbindgen:-internal}
 "

rust/Cargo.toml

@@ -24,6 +24,12 @@ path = "src/lib.rs"
 crate-type = ["staticlib"]
 
 [profile.release]
+# Unwinding across FFI boundaries is undefined behavior, and anyways, we're
+# [crash-only](https://en.wikipedia.org/wiki/Crash-only_software)
 panic = "abort"
-lto = true
+# We assume we're being delivered via e.g. RPM which supports split debuginfo
 debug = true
+# For true release builds, we do suggest you enable LTO via e.g.
+# env RUSTFLAGS='-C lto=true' as part of the outer build.  It's just
+# off by default because it's quite slow and the default is more oriented
+# towards local development.